Acoustic wave transducers are conventionally divided into bulk acoustic wave (BAW) and surface acoustic wave (SAW) devices. The majority of BAW devices employ a 0.2 to 0.5 mm thick AT-cut quartz resonator disc coated with metal electrodes, such as gold electrodes, on either side of the disc. A high frequency (low MHz) sinusoidal voltage is applied across the gold electrodes causing the quartz resonator disc to oscillate at its resonant frequency. When used as a mass sensor, this device is referred to as a quartz crystal microbalance (QCM). The quartz crystal microbalance has become widely used as a biosensor.
Piezoelectric material consists of atoms and/or molecules which all have their dipole moments aligned in the same direction within a lattice. If an outside force is applied to the lattice in such a way as to shift the alignment of the dipole field alignments, a voltage is produced. In the case of conventional QCM devices, the quartz crystal serves as the piezoelectric material, and the outside force comprises an alternating high frequency sinusoidal voltage applied to metal electrodes coated on the quartz crystal disc. The stringent conditions under which such quartz crystal discs are produced results in very reproducible discs and, therefore, reliable results.
However, conventional QCM acoustic transducers have a number of limitations. There is a strict requirement to photolithographically apply a metal film onto the disc of piezoelectric material. Additionally, hard wire connections to the metal film are required. Conventional QCM devices have a detection limit of approximately 1 ng/mL, which is inadequate for the monitoring of low molecular weight biomolecules. All of these problems impede the development of a practical acoustic sensor based on conventional QCM technology.
A new acoustic sensor, the magnetic resonance sensor (MARS), has recently been developed which offers an alternative to the QCM device. This technology has been described, for example, by Stevenson et al. in U.S. Pat. No. 5,869,748, issued Feb. 9, 1999. The MARS transducer described by Stevenson et al. establishes an acoustic resonance in a free-standing metallized silica glass plate using remote magnetic and electromagnetic fields. The device exploits magnetic fields for generation of acoustic waves in a thin metal film coated on one side of the silica glass plate. A coil connected to a RF generator, and a permanent magnet are placed on one side of the metallized silica glass plate. The magnet is not in direct contact with the plate and is thus said to be “remote” from the plate, although the induced magnetic fields extend to the plate. The magnetic fields achieve excitation of ions within the metallized coating on the plate. Unlike other previously designed electromagnetic-acoustic transduction sensors (EMATS), the transduction efficiency of the MARS device benefits from both electrical and acoustic resonance effects.
When exposed to an electromagnetic field, acoustic waves are produced in a metal film as a consequence of the radial Lorentz forces generated within the film. These “non-contact” forces are then conveyed, through momentum caused by contact of the metal film with a silica glass plate, to achieve acoustic resonance in the glass plate. The process is described by equation 1, where the Lorentz forcing term, F(z), is coupled to differential terms representing the elastic properties of the silica glass plate:
                                                                        ∂                2                            ⁢              u                                      ∂                              t                2                                              -                                    V              s                        ⁢                                                                                ∂                    2                                    ⁢                  u                                ⁢                                                                                              ∂                                  x                  2                                                                    =                              F            ⁡                          (              z              )                                C                                    (        1        )            where C is the elastic modulus of the silica glass plate; u is the particle displacement; and VS is the shear velocity. Because only one side of the glass plate is being driven, both the asymmetric and symmetric standing waves can be supported by a plate of thickness d, where the acoustic wave vector, k, is equal to ρm/d, where m is an integer. The resonance frequency, fR, can be calculated from the following equation:
                                                                        f                R                            =                                                m                  ⁢                                                                          ⁢                                      V                    S                                                                    2                  ⁢                  d                                                                                                        m                =                1                            ,              2              ,              3              ,              …              ⁢                                                          ,                              n                .                                                                        (        2        )            The resonance frequencies occur at harmonics of the fundamental frequency (m=1) and occur twice as often in a device such as the MARS device as compared to a QCM device.
Acoustic wave generation in the metallized silica glass plate is associated with a radio frequency generated in the coil, in the order of 10 s of mAs. The current gives rise to a series of voltage dips, on the order of mVs, at frequency intervals corresponding to the harmonic series of standing waves. The voltage dip corresponds to an acoustic resonance because the coil receives reflected RF power from the metal film that reduces in value when acoustic power is generated. The received signal voltage can be described by the following equation:
                    V        =                                                            GB                2                            ⁢                              IQ                e                                                    ρ              ⁢                                                          ⁢                                                V                  S                                ⁡                                  (                                      1                    +                    β                                    )                                                              *                      2                          α              ⁢                                                          ⁢              d                                                          (        3        )            where V is the received signal voltage; B is the magnetic field; I is the source current; Qe is the quality factor for the parallel resonant circuit; ρ is the density of the glass plate; VS is the shear velocity for the acoustic wave; α is the attenuation coefficient; d is the thickness of the plate; and β is an adjustment factor for phase differences that may exist across the metal film.
The MARS system offers advantages over the established QCM systems. From the above equation, it is clear that the received signal voltage can be increased through a variety of routes, such as by increasing the magnetic field strength, or by increasing the source current. An applicable source current frequency may range from the low MHz range up to around 60 MHz. However, the MARS system requires both a permanent magnet and electromagnetic field generation from the coil in order to induce appropriate movement within the metal film which then induces vibration in the plate.
The MARS device involves only indirect generation of vibration in the silica glass plate because only the metal film is initially caused to vibrate due to the magnetic and electromagnetic fields. The momentum from the vibration of the metal film is then imparted to the lattice of the silica glass plate. Thus, the glass plate is caused to vibrate only indirectly because of its proximity adjacent to the metal film. Because the sensing portion of a MARS device is indirectly caused to resonate through vibration of the metal film, a MARS sensor induces indirect generation of vibration in a sensor, and does not incorporate electromagnetism.
The above-described MARS device suffers from problems arising from reproducibility. Because resonance occurs in both the metal film and the silica glass plate, inconsistencies in the shape, thickness or density of either the film or the plate will effect the resulting vibration of the plate, and the shape of the acoustic resonance. The shape of the acoustic resonance for either symmetric or asymmetric modes can be effected. If an acoustic response does not appear to be a single peak, but rather as a doublet, at lower frequencies, or multiple peaks clustered around a main central resonance, this suggests that the glass plate faces are not parallel, or that they are acoustically isotropic. Inconsistencies in the plate complicates the results obtained from the MARS sensor because a shear wave generated in the metal film does not travel in a single dimension. Instead the glass plate supports the generation of lateral waves, requiring the incorporation of a more complex three-dimensional resonator model to account for the distorted resonance envelope. Thus, inconsistencies in plate shape, thickness or density introduces a significant amount of error when comparing the results obtained using different silica plates. From equation 3, it is clear that differences in plate thickness (d), non-parallel plate faces (β, VS, α) and plate density (ρ, α, VS) profoundly affect the received signal voltage.
Although the MARS device traverses the requirement of QCM systems to photolithographically apply a metal film electrode onto a specially polished crystal of piezoelectric material, metallization of the silica glass plate is still required, and new problems associated with reproducibility in the plate specifications are introduced.
Transverse shear mode acoustic wave sensors have been used in an increasing number of applications over the past number of years. The sensors employ a piezoelectric (usually quartz) disc as the transducing element. The sensors generate specific forms of mechanical resonance in the substrate, resulting in acoustic waves propagating in different directions. To do this, thin slices are cut from single crystal quartz (for example) at specific orientations with respect to the crystallographic axis. The geometry of the final slice defines the boundary conditions, while the orientation defines the values of the different matrices. When combined with the wave equations, they lead to solutions, which describe the different possible piezoelectric device structures and their behaviour1.
Sauerbry presented a relation between the amount of mass deposited onto a quartz crystal surface and its resonant frequency2. Now it is known that the quartz crystals are not only sensitive to mass, but also to coupling between the crystal and its surrounding environment3. To make the crystal sensitive to specific chemical species, coatings that bind or adsorb the analytes of interest may be applied.
The development of transverse shear mode sensors is currently impeded by a number of factors. For example, metal electrodes must be applied to the crystal, which increases the complexity of the chemistry required to immobilize selective films. Hardwire connections must be made to the electrodes, which may disrupt the flow of liquid through the cell. Ideally, detection and chemistry should be separated. Further, the detection limit of approximately 1 ng/mL is often inadequate for monitoring low molecular weight biomolecules. Additionally, there are difficulties involved in adapting such a sensor to work at higher frequency modes, in order to increase sensitivity. There is a need for a sensor that eliminates one or more of the above-noted problems.
It is, therefore, desirable to provide a sensor device which incorporates electromagnetic generation of vibration within a sensing portion of the device, and which is less susceptible to variability than the above-noted MARS technology.